So I have been playing Karthus lately and I've noticed he could deal serious AOE damage during teamfights and carry many games. So here's my build for Carry Karthus. This build can be used in ranked play and casual.

Runes are flexible. Any caster setup will do. The runes stated above are the ones that I run in my setup. Magic pen are a must for marks, seals are flexible and also glyphs. You could sacrifice durability for more damage potential. It's your choice.

Masteries are standard caster setup 9/0/21, taking magic pen in Offensive and improved flash and movement in Utility. Not much more to say.

Start off with a Doran's ring, farm up until you have enough for tear of goddess and basic boots. Build Archangels then Rabadon's. Void staff is a must for the magic pen. Late game go with either Rylai's for survivability or pure damage with another Archangels. I've had 1.3k AP before with 3 Archangels.

Flash and ignite. Flash is probably the best spell, both offensively and defensively. There's other choices other than ignite. Exhaust against heavy dps teams. Ignite works charms getting that early kill in lane and also destroying many different champions such as Irelia, Singed, Mundo, Warwick, Olaf, Mordekaiser and more. Spells like teleport and clarity are useless. Ghost is another strong choice though I don't use it for Karthus.

Well, your off either to mid lane or solo top. Never should Karthus have to lane with someone else. Karthus needs to farm so spam your Lay Waste. Don't worry about mana issues early game. Harass with Lay Waste. A well placed Q can take a whole bar off of most champs in mid. Once you reach lvl 6, map awareness is key. Use F2, F3, F4, F5 keys to check around the map if anyone is low on hp.

WARNING! : Before you use your ultimate, always factor in magic resist and shields (shen and friends). I've seen many Karthus' use their ults just because someone is low but they don't get a kill because he/she didn't do their math. Never waste thy requiem. It has a very long cooldown.

This is where Karthus shines the most especially using this build. I've seen most Karthus' just hang around in the back spamming Q. For the most part, that works but in this build it isn't the case. For this build, you will be initiating right after the tank soaks the first few attacks. You will provide constant AOE damage with Defile and Lay Waste. Using your Wall Pain, none of the enemies should be allowed to escape with their life. If they do, you always that notorious ultimate of yours. If you die, make sure you use your Ultimate right away. That way you provide enough damage for your team to mop up the mess.

Well that's it! Remember when playing Karthus that map awareness is the key. Early game kills will greatly benefit you mid to late game. Farming is another factor in your success. Spam Q like no tomorrow and soon enough your mana pool will grow thus increasing your AP. Teamfights, your the flag carrier, staying right behind the tank focusing down hard carries and microwaving your enemies with your Defile (It's like having a Fiddlesticks ultimate 100% of the time!). And if you so happen to die, just hit "R" and watch the "All Chat" fill with rage. I usually just end up replying "Umadbrah?". :)

I hope you enjoyed reading my first guide and I wish you luck on the fields of Justice as the Deathsinger.